Nips are typically employed at several stages in the manufacture of paper products such as bath tissue and paper toweling. In practice, a web material is passed through these nips to form the web material into the intended paper product. Examples of these nips may include dewatering presses located in paper machines, extended nips, calendaring nips, as well as the nips provided in the various winders. Nips are provided in these operations to provide desirable characteristics into the intended product. By way of non-limiting example, in a dewatering press, the transverse distribution (in the axial direction of the nip rolls) of the nip pressure affects the transverse moisture profile of the web to be pressed.
Another example to demonstrate the use of nips in a manufacturing operation is a nip associated with the reel-up process of a paper winding operation. Here, the process can begin with an empty spool or reel core that is brought into contacting engagement with a reeling cylinder—typically on a pair of rotating arms that terminate in forks that extend on either side of the reel core bearings. Once the paper reel has reached a given size, the roll spool is positioned between a pair of carriages which ride on level rails. Web tension is controlled by the reeling cylinder and torque is applied to the reel spool by a center wind assist. Nip load is controlled by hydraulic cylinders that position the carriages on which the bearing housings and thus the paper reel are supported. The hydraulic cylinders adjust the position of the paper reel to control the nip loading of the paper reel with the reeling cylinder. Nip pressure may be monitored by monitoring the pressure in the hydraulic cylinders which position the carriages.
In any regard, it should be understood that nips used in the consumer paper products industry commonly utilize a set of rolls (two or more) that are loaded to (i.e., pressed against) one another. Generally, it is desirable to load these rolls to one another at a set force. One of skill in the art will recognize this to be known generally as the nip force and is generally provided (or referenced) in terms of force per unit length. By convention herein, the units are known as pounds per linear inch (PLI or pli).
Generally, there are two methods to set the loading force between contacting cylinders. These techniques are known individually as “loading to pressure” and “loading to stops.” The process of “loading to pressure” generally utilizes the force of lifting cylinders to go only to lifting a roll into place and then applying a load between the rolls. The process of loading to stops provides a lifting cylinder that provides a force to lift a pivoting roll. The applied lifting force presses the roll's bearing housings against a stop mechanism. The stop mechanism can be adjusted to control the amount of force seen between the contacting rolls.
More specifically, the process of loading to pressure can be simply described. In this method when the pivoting roll is commanded to load, hydraulic pressure is introduced into the load cylinder. The pressure introduced to the cylinder is set to provide a specific load, measured in PLI, between the two rolls the amount of pressure required is calculated via a free body diagram of the system and can be confirmed by measuring the nip width between the rolls. This is suitable when one or both rolls are rubber covered. In cases where both rollers are hard covered, a pressure sensitive film can be used to determine the nip force.
In these described prior art methods, problems have been encountered in the associated devices used for the measurement of nip forces relating to the calibration of the detectors in the transfer of the signal from the rotating roll. For example, the transfer of the signal can be accomplished through the use of glide rings and equivalent arrangements as well as telemetry equipment. However, these devices are complicated and susceptible to disturbance.
Thus, it would be advantageous to provide a novel device and method for measuring and controlling the nip forces and pressures between two rolls such as those used in an embossing process or a calendaring process. It would also be advantageous to provide a novel device and method to effectively distribute the nip forces and/or pressures between two rolls used in the manufacture of products such as consumer paper products. Along these lines it would also be advantageous to provide a measurement device and method that is suitable for on-line measurement of nip forces and/or nip pressures during production operation. It was also be advantageous to provide a device and method in which the problems related to the placement of detectors on a nip roll or nip band are minimized. Additionally, it would be advantageous to provide a device and method that provides for an easier and more accurate calibration of detectors than is currently known and available to those of skill in the art. It is envisioned that the drawbacks discussed above can be substantially avoided and the advantages realized by use of the apparatus disclosed herein.